System and method for clamp current regulation of induction machines

ABSTRACT

A device to regulate current produced by an induction machine responsive to a plurality of phase current signals. The motor produces torque for application on a shaft. A processing and drive circuit responsive to a direct current command signal and a quadrature current command signal produces phase current signals for input to the motor. A command circuit responsive to the phase current signals, an angular position of said shaft, and a voltage input command signal to produce a direct current error signal and a quadrature current error signal. A control circuit responsive to the direct and quadrature current error signals produces the direct voltage signal command and the quadrature signal command. The control circuit has a direct and quadrature proportional gain, integrator and clamp circuits. An algorithm produces limited or clamped voltage modulation index signals to obtain maximum efficiency and maximum torque per ampere in the speed range.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention is directed generally to electromechanicalmachines, and, in particular, to a system and a method for currentregulation in the field-weakening operation of induction machines.

2. Description of the Related Art

In the control of inverter-driven induction machines, field weakening isoften used to lower the inverter voltage rating for a given application.Without field weakening the inverter manufacturer would have to usecomponents rated to handle higher levels of voltage for that givenapplication. This would undesirably add incremental costs to the drivesystem. During high speed operation, the phase current may be applied tothe machine windings in advance of the phase electromotive force. Tocontrol the electromotive force, the D axis current is decreasedinversely with speed.

It is known to provide a flux weakening algorithm by the use of a numberof look-up tables to produce the reference Q-axis and D-axis currents.However, the use of look-up tables requires the creation of numerous andcumbersome data structures within the look-up tables themselves tohandle all possible situations in the system and its environment.

Accordingly, there is a need for a current control system that minimizesor eliminates one or more of thc above-mentioned shortcomings.

SUMMARY OF THE INVENTION

The invention is based upon the discovery that in current control forinduction machines, a limiter in the current regulator of a proportionalintegral control that clamps the current regulators at the availablevoltage vector with appropriate phase preventing loss of currentregulation, maximizes the efficiency of the machine and produces maximumtorque in the field weakening region. The arrangement is capable ofadjusting to the true base speed of the machine as the DC-link voltagevaries.

The invention is directed to a device for regulating current produced inan induction machine responsive to a plurality of phase current signalsfor producing torque at the machine output shaft. The device includes aprocessing and drive circuit responsive to a direct current commandsignal and a quadrature current command signal for producing a pluralityof phase current signals at the input to the machine. A command circuitis responsive to the phase current signals and an angular position ofthe shaft for producing a direct current error signal and a quadraturecurrent error signal. A control circuit is responsive to the direct andquadrature current error signals to produce the direct and quadraturevoltage modulation index signals. The control circuit includes aproportional controller and integrator responsive to each of the directand quadrature current error signals. The proportional controller andintegrator outputs are summed and fed back to the command circuit. Alimiter is located in the feed back loop to clamp the summed outputs atthe available voltage vector and provide appropriate phase.

The method according to the invention limits current regulators to anavailable voltage magnitude, voltage angle delta and a slip frequencyrange for assuring that the system does not exceed the available voltageand that the machine efficiency is maximized and that maximum torque isproduced in the field-weakening region.

Other features, objects and advantages of the present invention willbecome apparent to one of ordinary skill in the art from the descriptionthat follows and may be realized by means of the instrumentalities andcombinations particularly pointed out in the appended claims, taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of an induction machine driveemploying exemplary control system according to the invention.

FIG. 2 illustrates an exemplary current clamp command employed in thecircuit of FIG. 1.

FIG. 3 is a flow chart showing the implementation of a limitingalgorithm employed in the circuit of FIG. 2.

DESCRIPTION OF THE INVENTION

The invention is directed to a method and apparatus for current controlin induction machines. The method is specifically designed to produceadditional functionality in the field-weakening region.

To extend the operational speed range of induction machines, it isnecessary to de-flux the machine by applying reduced current in thesynchronous D-axis, Ids. The advantage of the method is that it appliesthe appropriate amount of current at each operating point across theentire speed range of the machine, that is, in the constant torqueregion and in the field-weakening region.

An important feature of the invention is that when the current regulatorexceeds the available voltage, the system decreases Ids. Furthermore,when the voltage angle exceeds the maximum voltage angle delta formotoring mode, or is less than the minimum voltage angle delta forgenerating mode, the system clamps the angle to the maximum delta anglefor motoring mode or the minimum delta angle for generating mode, andthe slip frequency is kept to the value that provides maximum torque formotoring or minimum torque for generating mode. As a result, the currentregulator does not run out of voltage, maximizes the machine efficiencyand provides maximum torque in the field-weakening region for motoringmode, or minimum torque in the field weakening region for generatingmode.

It is important to note that the base-speed of a machine changessignificantly depending upon variations in the DC-link voltage. However,the invention is able to properly identify the base-speed so that theconstant torque region is extended to as high a speed as possible.

FIG. 1 shows a block diagram of an exemplary system 10 according to theinvention. The apparatus is adapted for controlling an induction machine12, e.g. a motor/generator, having a stator S and a rotor R for drivinga shaft. The machine is driven by a three phrase inverter 14 coupled toa DC-link voltage source 16 (Vdc). Vdc is sometimes herein afterreferred to as the link voltage. A pulse width modulator (PWM) 18 drivesthe inverter in a known way.

Control of the induction motor may be implemented by a digital signalprocessor (DSP). Such DSPs are known and are arranged to be responsiveto various inputs for producing control outputs, for driving the motoraccording to the invention. A sensor Sθ is coupled to the motor 12 toproduce a sensor rotor position signal θ_(r). The sensed rotor positionsignal θ_(r) is coupled to a theta synchronous block 20 which producessynchronous angle signal θ_(e). The output of block 20 is coupled tocoordinate transform circuits 22 and 24, as shown. The coordinatetransform 22 transforms D-axis and Q-axis modulation index signals toproduce modulation index signals in stationary coordinates α and β. Themodulation index signals in the stationary coordinate frame are coupledto and modulated by a space vector modulator 26 in a known manner toproduce outputs that drive the voltage PWM modulator 18. PWM modulationblock 18 generates the gate drive signal for inverter 14, which providesvoltage to machine 12.

Motor drive phase currents a, b and c to the machine 12 from inverter 14drive the motor 12. These are coupled in feed back relation to thecoordinate transform 24 which transforms motor drive stationary currentfrom the inverter 14 to direct and quadrature synchronous axis signalsIdsf and Iqsf. These signals are likewise coupled to the thetasynchronous block 20 as shown, and to adders 34 and 36 in the currentclamp regulator circuit 30.

The current clamp regulator 30 includes an Ids current command block 32and an Iqs current command block 33 which calculate direct andquadrature current commands Ids* and Iqs* respectively (Commands aredesignated by an asterisk (*)). The current commands Ids* and Iqs* aresummed with the respective transformation feedback output Idsf and Iqsfat summing nodes 34 and 36 respectively. Ids* and Iqs* are each coupledto the non inverting inputs of respective summing nodes 34 and 36.Likewise Idsf and Iqsf are coupled to the inverting inputs of respectivenodes 34 and 36. The summed signals respectively represent the D-axiscurrent error signal Id error and the Q-axis current error signal Iqerror. The error signals are coupled to correspondingproportional-integral (PI) current regulators 38 and 40. The outputs ofthe PI regulators represent the D-axis voltage Vds and the Q-axisvoltage Vqs respectively. These signals are coupled to the transformcircuit 22 for appropriate transformations as noted above. The voltagesignals Vds and Vqs are also fed back to the Ids current command block32 as shown and are coupled to the theta synchronous block 20.

The PI current regulators 38 and 40 include limiters for clamping theD-axis and Q-axis modulation index signals Vds and Vqs to some limitedvoltage with a given limited angle delta in order to prevent anundesirable loss of current regulation, to maximize machine efficiencyand to provide maximum torque in the field weakening region.

The current command circuit 32 receives a Torque* command input, and azero vector time control command signal T0* as an input. Alternatively,the current command circuit 32 may receive the addition of time 1 andtime 2 vectors as a control signal, i.e., (T1+T2)* or the voltagemagnitude command signal V_(MAG)*. The Ids current command circuit 32receives a feedback signal corresponding to the zero vector time T0 fromspace voltage modulator 26. Feedback signal from the space modulator 26may be in the form of the sum of vectors 1 and 2 i.e. (T1+T2) oraccording to a known relationship, namely V_(MAG)=√{square root over(Vds²+Vqs²)}.

FIG. 2 illustrates a block diagram of the clamp arrangement employed inthe proportional integrators 38 and 40. As illustrated, for the D-axis,the zero time vector T0 is fed back from the space vector modulator 26and is input to a ripple filter 42. The output of filter 42 is time zerofeedback output T0 f which is summed at the non-inverting input of node44. The zero time vector control T0* is coupled to the inverting inputof the node 44. The difference is error signal T0 e which is coupled toa proportional integrator (PI) circuit 46 and which feeds limiter orclamp 48. The clamp 48 has a feedback loop of 50 which is coupled to thePI circuit 46 as shown.

The limiter 48 produces an output IdsT0 which is limited to values lessthan or equal to zero.

The output IdsT0 of the limiter 48 is summed at node 54 with a look uptable (IdsLUT) output for Ids from maximum torque per amp curve block 52both of which are non-inverted, as shown, to produce the D axis currentcontrol signal Ids*. Block 52 is fed by the Torque* control signal.

The Ids* signal and Idsf feedback signal arc summed at the respectivenon-inverting and inverting inputs of node 34 to produce Id error, whichis coupled to parallel connected proportional circuit 56 and integratingcircuit 58. Proportional circuit 56 produces one output which is coupledto the non-inverting input of summing node 60. The integrating circuit58 also coupled to the node 34 is, in turn, coupled to a clamp orlimiting circuit 62 having a feedback loop 64 as shown.

The proportional circuit 56 controls the transient components of the Iderror signal, and the integrating circuit 58 controls steady statecomponents of the Id error signal. The clamp 62, when implemented, isused to limit the steady state value within an allowable range. Theoutput of the clamp 62 is coupled to another non-inverting input of thesumming node 60. The output of the node 60 is the unclamped D-axismodulation index signal Vds. This signal is coupled to clamp or limitingcircuit 66, and when engaged, the output of the clamp 66 is the clampedD-axis modulation index signal Vds. As shown in FIG. 1, the Vds signalis coupled to the transform circuit 22 and is fed back to the currentcommand circuit 32 and the theta synchronous block 20. The second clamp66 limits the overall output Vds.

Iqs current command block 33, FIG. 2., produces Iqs* command signal inresponse to the torque command signal Torque* and feedback signal Idsffrom transform block 24. The Q-axis reference or command signal Iqs*,FIG. 2, is coupled to the non-inverting input of the node 36. The Iqsffeedback signal from the transform circuit 24 is coupled to theinverting input of the node 36. The node 36 produces an Iq error signal.The Iq error signal is coupled to proportional controller 70 and theintegrator 72. Proportional circuit 70 produces transient signals andthe integrating circuit 72 produces steady state signal. The output ofthe proportional control 70 is coupled to the non-inverting input of anode 74. The output of the integrator 72 is coupled to a clamp 76. Theclamp 76, when implemented, is used to limit the steady state valuewithin an allowable range. The output of the clamp 76 is coupled toanother non-inverting input of summing node 74 and is fed back over line78 to integrator 72, as shown. The signals are summed at node 74, andthe output of the node 74 is the unclamped Vqs. This signal is coupledto clamp or limiting circuit 80, and when engaged, the output of theclamp is clamped Q-axis modulation index signal Vqs. This, in turn, iscoupled to the transform circuit 22 and is fed back to the currentcommand circuit 32 and the theta synchronous block 20 as shown in FIG.1. The second clamp 80 limits the overall output Vqs.

When activated, clamps 66 and 80 limit the overall D and Q axis signals.Clamps 62 and 76 limit or clamp the steady state signals. The limitingor clamping circuits illustrated in FIG. 2 thereby result in theprotections and versatility afforded by the circuit of the presentinvention.

Activation of clamps 66 and 80 occurs when the unclamped Vds, incombination with unclamped Vqs is out of a selected voltage vectorrange. i.e. magnitude and direction (delta angle). Once engaged, clamps66 and 80 corresponding clamps 62 and 76 are operative through feedbacklines 67 and 81 to implement an algorithm exemplified in FIG. 3 to limitVds and Vqs to the clamped values as shown.

FIG. 3 is a flow chart which illustrates a flow chart for implementingan algorithm for initiating clamping action of the apparatus of thepresent invention. The unclamped direct and quadrature voltage signalsVds and Vqs are used to calculate the delta angle in block 80. The deltaangle is (arc tan (−Vds/Vqs)). The delta angle signal of block 80 iscompared with a delta max lookup signal of block 82 in comparator block84. The delta max look-up signal is derived by examining the rotor speedWr produced by differential circuit 86 in response to the rotor positionsignal θ_(r), as shown.

If the delta angle is greater than or equal to the delta max, as sensedat block 84, then Vds and Vqs are recalculated via blocks 110, 88, 90and 92, as shown. The output of block 92 represents the clamped D and Qaxis signals. The function of blocks 110 and 88 are discussed below. Ifthe delta angle is less than the delta max, as determined at block 84,then a No Change output is produced at block 94. The No Change output iscoupled to the block 92 which produces Vds and Vqs outputs which areunclamped.

As further illustrated in FIG. 3, if the delta angle is greater than thedelta max, as determined in block 84, then the Slip Frequency Max. lookup block 96 provides the slip frequency Wslmax in response to the rotorspeed signal Wr derived from differentiating block 86. The maximum slipfrequency of block 96 sets the value in block 98 and inputs this toblock 104 for summation with the rotor frequency Wr. If the delta angleis not greater than delta max (Block 84), then the slip frequency Wsl iscalculated from feedback currents Idsf and Iqsf via blocks 100 and 102.The slip frequency is added to rotor frequency to obtain synchronousfrequency We in block 104. The output of 104 is integrated at 106 toproduce a synchronous position output signal θ_(e). The synchronousposition signal θ_(e) is employed as an input to transform circuits 22and 24 (FIG. 1).

In addition, block 110 may be responsive to the affirmator output ofDelta max block 84. Block 110 determines if the Iq error signal isgreater than or equal to a minimum value of the Iq error. An affirmativecomparison is coupled to block 88 where Delta Angle is set to Delta MaxAngle which is supplied from block 82. Vds and Vqs are recalculated inblock 90 and output at block 92 as noted above. A negative comparison iscoupled to block 94 where upon Vqs and Vds experience No Change.

The foregoing arrangement is for a motoring mode. Accordingly, if themotor goes to the generating mode, delta and slip frequencies changesign and the comparison becomes a less than calculation in blocks 84 and110 of FIG. 3

From the foregoing, it can be seen that a new and improved device toregulate current produced by an induction machine has been provided. Itis to be understood that the description of the exemplary embodiments ismerely illustrative of some of the many specific embodiments thatrepresent applications of the principles of the present invention. Otherarrangements would be evident to those skilled in the art withoutdeparting from the scope of the invention as defined by the followingclaims.

1. A device to regular current produced by an induction machine comprising: an induction motor responsive to a plurality of phase current signals, said motor producing torque for application on a shaft; a processing and drive circuit responsive to a direct current command signal and a quadrature current command signal, said processing and drive circuit for producing a plurality of phase current signals for input into said induction motor; a command circuit responsive to said plurality of phase current signals, to an angular position of said shaft, and to a current input command signal for producing a direct current error signal and a quadrature current error signal; a control circuit responsive to the direct and quadrature current error signals for producing direct and quadrature voltage signals; and a limiter for limiting the direct and quadrature voltage signals to a selected level.
 2. The device of claim 1 including a D-axis current command circuit responsive to a torque command, Torque*, one of a zero time vector, T0*, a time for voltage vector, T1+T2*, and a voltage magnitude Vmag* and one of a feedback for zero time vector, T0, time for voltage vector, T1+T2, and voltage magnitude Vmag.
 3. The device of claim 1 further including a transform circuit responsive to phase current signals of the drive circuit to produce direct and quadrature synchronous current feedback signals Idsf and Iqsf.
 4. The device of claim 1 including a Q-axis current command circuit responsive to a torque command, Torque*, and a feedback D-axis current, Idsf, for producing a Q-axis current command.
 5. The device of claim 1 wherein said command circuit includes, a control circuit responsive to said direct and quadrature current error signals to produce said direct voltage command signal and said quadrature command signal, said control circuit having a first portion and a second portion, wherein said first portion includes a first pathway having a first proportional gain and a second pathway having a first integrator and a first clamp, and a third pathway having a first summing node and a second clamp, wherein said second portion includes a fourth pathway having a second proportional gain and a fifth pathway having a second integrator and a third clamp and a sixth pathway having a second summing node and a fourth clamp.
 6. The device of claim 5 wherein said first portion produces said direct voltage command signal, and said first and second clamps contain said voltage Vds MIN.≦Vds≦Vds MAX.
 7. The device of claim 5 wherein said second portion produces said quadrature voltage command signal, and said third and fourth clamps contain said voltage Vqs MIN.≦Vqs≦Vqs MAX.
 8. The device of claim 1 wherein said direct current error signal is produced by subtracting a direct synchronous current feedback signal from a direct synchronous current command signal.
 9. The device of claim 2 further including a proportional integrator responsive to a zero time vector feedback T0 and a zero time vector command T0* for producing an output, and a limiter responsive to the output for producing a D-axis time zero signal, IdsT0, between zero and a selected value; a maximum torque per ampere circuit for producing a D-axis current look up signal, IdsLUT; a summing circuit responsive to the D-axis time zero signal and the D-axis current look up signal to produce a D-axis current command Ids*.
 10. The device of claim 1 further comprising in a motor mode means for determining the angle representing a sensed direction of the vector, means for determining a maximum angle of said vector and means for recalculating the voltages to a limited value based on the maximum angle and setting the slip frequency to a maximum value which is a function of the rotor speed, when the sensed angle exceeds the maximum angle.
 11. The device of claim 1 further comprising means for calculating the slip frequency as a function of direct and quadrature synchronous current feedback signals.
 12. The method of claim 1 further comprising means for determining in a generating mode the angle representing a sensed direction of the vector, means for determining a minimum angle of said vector and means for recalculating the voltages to a limited value based on the minimum angle and setting the slip frequency to a minimum value which is a function of the rotor speed, when the sensed angle is less than the minimum angle.
 13. A method to regulate current produced by an induction machine comprising limiting each of the respective direct and quadrature voltages Vds and Vqs to selected value range which together represent the magnitude and direction of a resulting voltage vector.
 14. The method of claim 13, wherein the vector comprises √{square root over (Vds²+Vqs²)}, or K(1−T0), or K(T1+T2).
 15. The method of claim 13 wherein the direction of the vector comprises arc tan−Vds/Vqs, the voltage angle relative to Q axis.
 16. The method of claim 13 further comprising calculating a synchronous angle θ_(e) by differentiating a rotor position signal to derive rotor speed, selecting a maximum slip frequency based on the rotor speed, adding the rotor speed and slip frequency to produce synchronous frequency speed and integrating the synchronous frequency to produce a synchronous position angle θ_(e).
 17. The method of claim 13 further comprising determining in a motor mode the angle representing a sensed direction of the vector, determining a maximum angle of said vector and recalculating the voltages to a limited value based on the maximum angle and setting the slip frequency to a maximum value which is a function of the rotor speed, when the sensed angle exceeds the maximum angle.
 18. The method of claim 13 further comprising calculating the slip frequency as a function of direct and quadrature synchronous current feedback signals.
 19. The method of claim 13 further comprising determining in a generating mode the angle representing a sensed direction of the vector, determining a minimum angle of said vector and recalculating the voltages to a limited value based on the minimum angle and setting the slip frequency to a minimum value which is a function of the rotor speed, when the sensed angle is less than the minimum angle.
 20. A method to regulate current produced by an induction machine comprising the steps of: inputting a direct current command signal and a direct current feedback signal into a first circuit; subtracting said direct current feedback signal from said direct current command signal to produce a first direct current error; inputting said direct current error signal into a first proportional controller to produce a first direct voltage signal command; inputting said direct current error signal into a first integrator to produce a second direct voltage signal command; inputting said second direct voltage signal command into a first limiter to produce a third direct voltage signal command; adding said third direct voltage signal command with said first direct voltage signal command to produce an unclamped direct voltage signal command; inputting said unclamped direct voltage signal command into a second limiter to produce a direct synchronous voltage signal command; inputting a quadrature current feedback signal and a quadrature current command signal into a second circuit; subtracting said quadrature current feedback signal to said quadrature current command signal to produce a quadrature current error signal; inputting said quadrature current error signal into a second proportional controller to produce a first quadrature voltage signal command; inputting said quadrature current error signal into a second integrator to produce a second quadrature voltage signal command; inputting said second quadrature voltage signal command into a third limiter to produce a third quadrature voltage signal command; adding said third quadrature voltage signal command with said first quadrature voltage signal command to produce an unclamped quadrature voltage signal command; and inputting said unclamped quadrature voltage signal command into a fourth limiter to produce a quadrature synchronous voltage signal command.
 21. The method of claim 20 wherein said third direct voltage signal command represents a steady state value and said first limiter clamps the value of said third direct voltage signal command to be no greater than a selected maximum value and to be no less than a selected minimum value.
 22. The method of claim 20 wherein said second limiter clamps the value of unclamped said direct voltage signal command to be no greater than a selected maximum value and to be no less than a selected minimum value.
 23. The method of claim 20 wherein said third direct quadrature voltage signal command represents a steady state value and said third limiter clamps the value of said third quadrature voltage signal command to be no greater than a selected value and to be no less than a selected minimum value.
 24. The method of claim 20 wherein said fourth limiter clamps the value of said unclamped quadrature voltage signal command to be no greater than a selected value and to be no less than a selected minimum value.
 25. A device to regulate current produced by an induction machine comprising: an induction motor responsive to a plurality of phase current signals, said motor producing torque for application on a shaft; a processing and drive circuit responsive to a direct current command signal and a quadrature current command signal, said processing and drive circuit for producing a plurality of phase current signals for input into said induction motor; a command circuit responsive to said plurality of phase current signals, to an angular position of said shaft, and to a current input command signal for producing a direct current error signal and a quadrature current error signal; and a control circuit responsive to said direct and quadrature current error signals, said control circuit having a first portion and a second portion, wherein said first portion includes a first proportional integrator and a first clamp means connected thereto, for producing the direct voltage signal command, said second portion includes a second proportional integrator and second clamp means connected thereto for producing the quadrature direct voltage signal command.
 26. A method for regulating current produced by an induction machine driven by direct and quadrature signals comprising the steps of: subtracting a direct current signal from a direct current signal command to produce a first direct error signal; proportionally integrating the direct error signal to produce the proportionally integrated direct signal command; limiting the proportionally integrated first direct signal command to produce the direct signal command; subtracting a quadrature current signal from a quadrature current signal command to produce a quadrature direct error signal command; proportionally integrating the quadrature signal command to produce a proportionally integrated quadrature signal command; limiting the proportionally integrated quadrature signal to produce a quadrature signal command. 